This invention relates to an image suppression mixer of the waveguide type which not only has the effect of image suppression but also prevents leakage of local oscillation signals to signal circuits through a simple circuit structure.
Recently, satellite broadcast systems have been introduced in various countries, with the development of various kinds of satellite broadcast receivers. The important features of such satellite broadcast receivers are low noise, efficient image suppression, low leakage of local oscillation and so forth. The low feature determines the receiving sensitivity of the receiver, while the image suppression effect and low leakage of local oscillation affect interference between ground communication lines and satellite communication systems. External objectionable signals and noise of image frequencies of the receiver applied to the receiver impair the signal to noise ratio during reception and demodulation, and local oscillation signals leaked into the receiver serve as interference with other communication systems.
Though various attempts have been made to improve the characteristics of the receiver, such improvement should be particularly simple and inexpensive, especially in the case where the receiver is used for a specific purpose.
One of the components which predominantly determine the major characteristics of the receiver is a receiver mixer. As is well known in the art, the function of the mixer is to mix satellite broadcast and local oscillation into an intermediate frequency (IF). A receiver mixer which is very simple in structure but excellent in performance will be of importance for practical use.
There are two types of conventional receiver mixers, an MIC mixer having strip lines, and a waveguide type mixer.
The waveguide type mixer is generally bigger in shape than the MIC type but much better in performance, having lower circuit loss thanks to a higher Q value. This advantage is more significant at higher frequencies.
The following is directed to a conventional waveguide type as shown in FIG. 4, because the present invention is interested in an improvement in the waveguide type mixer. In FIG. 4, the mixer includes a signal input terminal 1, a local oscillation input terminal 2, a band pass filter 3 for passage of signal frequencies, a band pass filter 4 for passage of local frequency, an IF signal output terminal 5 and a mixer diode 6.
Signals are fed through the terminal 1 and then applied to the diode 6 through the filter 3. Local oscillation is fed through the terminal 2 and applied to the diode 6 by way of the filter 4. Consequently, IF signals are delivered through the output terminal 5.
The property of the filter 3 blocks signals of image frequencies coming through the terminal 1 and suppress image components. Further, the property of the filter 3 prevents local oscillation components from leaking into the signal terminal 1. Image components generating at the mixer diode 6 on the other hand are blocked through the filters 3 and 4 and usually returned to the diode 6 (image recovery operation).
The feature of this structure is low conversion loss at the mixer because of its implementation, with relatively low circuit loss. However, it is large and complicated in structure and not suited for mass production.
A mixer of solid plan circuit is well known as another sort of the waveguide type mixer which has low conversion loss and is low noise and suitable for mass production.
FIGS. 5(a) and 5(b) illustrate the above metioned sort of the receiver mixer wherein a metal plate is inserted in the direction of signal propagation at the center of a wider side of a linear waveguide and provided with a local frequency band pass filter, a trap filter and so forth thereon. FIG. 5(a) is a side view thereof and FIG. 5(b) is a top view.
The mixer shown therein comprises a trap (block) filter 7 for local oscillation frequency or image frequency and a dielectric trap (block) filter 8 for an improved characteristic in preventing local oscillation from leaking into the signal terminal 1, as well as including the signal input terminal 1, the local oscillation input terminal 2, the local oscillation frequency band pass filter 4, the IF signal output terminal 5 and the mixer diode 6.
As in the type of mixer of FIG. 4, signals are fed through the terminal 1 and local oscillation is fed through the terminal 2, and IF signals generating at the mixer diode 6 are delivered through the terminal 5.
Image frequency components fed through the signal terminal 1 are blocked with the image trap filter 7 and substantially prevented from reaching the diode 6 due to short-circuit load at image frequencies generating at the location of the diode 6, which load is caused by the property of the local oscillation band pass filter 4. As mentioned previously, this sort of mixer has excellent properties such as low conversion loss and low noise and appropriateness for mass production. However, if more effective image suppression and lesser leakage of local oscillation into the signal terminal are desirable, precise and complicated design and adjustment are required for the dielectric local oscillation trap filter 8 itself, the characteristics of the image trap filter 7 and the local oscillation filter 4 and electric arrangement of the diode 6 in relation with those filters. This renders design and manufacture of the mixer difficult.
In some cases, a multi-stage trap filter is required with the results in a higher degree of complexity and deteriorated performance characteristics.